In the formation of paper, wood fibers are dispersed in water to form a papermaking stock. The stock is usually at least 99 percent water and contains one-half to one percent paper fibers. The paper stock is injected through a tapered flow control channel known as a slice onto a fourdrinier moving wire screen to form the paper web. In some circumstances the stock is injected between two moving wire screens on a so-called twin wire machine. Water is drawn from the stock through the forming screens or wires leaving a web of paper fibers which is pressed and dried to form a web of paper.
Modern papermaking machines are between one and four hundred inches wide and operate at speeds up to and in excess of 4,000 feet per minute. Thus, the headbox and the slice which supply the paper stock which is formed into the paper web must supply not only a large quantity of stock to meet the high forming speeds of modern papermaking processes, but also supply the stock extremely uniformly if the sheet of paper formed is to be of uniform thickness across the width of the web.
To achieve the high flow rates and uniformity of stock injected through the slice, the stock is pumped at extremely high pressures by means of pumping equipment. An attenuator is disposed upstream relative to the headbox for damping pressure pulses caused by the stock pumping equipment. The arrangement is such that the rate of stock entering the headbox is relatively constant.
To achieve a uniform flow of stock onto the forming wire or wires, the headbox employs an inlet header or manifold which is of a tapered configuration. Between the inlet header and the slice are a plurality of distributor tubes which are arrayed in a tube bank. The tube bank is typically in the neighborhood of six tubes high by several hundred tubes long. The stock flows from the tapered tube inlets through each tube disposed within the tube bank. It is essential that the rate of flow of stock through each distributor tube be uniform in order that the stock exiting the lips of the slice be uniform from one edge of the forming wire to the other.
In order to achieve such constant flow rate, the inlet header or manifold is tapered in the cross-machine direction. In other words, the width of the manifold in the machine direction decreases further away from the stock inlet. The cross-sectional area of the inlet header at its narrowest is equal to the cross-sectional area of the inlet header at the stock inlet less three times the total area of the tubes opening off the header. As the flow of stock moves down the tapered header, a portion of the main flow is diverted through the tubes. Therefore, the cross-sectional area of the header is reduced as it moves in the cross-machine direction so that its area remains substantially equivalent to three times the cross-sectional area of the tubes not yet reached by the header. Thus, the cross-sectional area of the header is decreased in order to compensate for the loss of fluid volume as paper stock flows from one side of the header to the other. This change in cross-sectional area maintains the same pressure in the header in the cross-machine direction which in turn maintains the same flow through the tubes in the cross-machine direction.
Consequently, the rate of flow of stock through all of the tubes in the cross-machine direction is maintained substantially constant. However, in practice the consistency has not been sufficiently uniform to prevent some variation in paper weight or thickness in the cross-machine direction. Thus, in some paper forming headboxes actuators on the lip of the slice have been used to deform the slice lip to change the width of the slice opening in an effort to maintain a uniform paper weight across the paper web. In one recently developed system, described in U.S. Pat. No. 5,196,091 to Richard E. Hergert and incorporated herein by reference, the injection of diluting water into the headbox header or manifold adjacent to the tube inlets has been used to control the dilution of the stock in the cross-machine direction. This dilution control in turn acts to control the paper web weight or thickness. This technique in fact has resulted in the production of paper webs of more uniform characteristics.
The stock from which paper is formed contains not only paper fibers but various additives designed to improve or facilitate the production of the paper web. These additives include fillers such as clay which increase the opaqueness of the paper. Other additives include long chain polymers which aid in the retention of the filler within the paper web. Other materials combined with the stock include softening agents used with certain grades of tissue paper. Additionally, additives may be supplied which facilitate the bonding of fibers to one another, for example the starch. In the existing process for forming paper, these additives are added well before the headbox inlet header and are uniformly mixed with the stock.
Thus, while the addition of chemicals or fillers is often necessary for the formation of a particular paper web, current methods of dispersing the chemicals in the paper forming stock may be less effective than desirable because many of the additives are high molecular polymers which break down under the application of fluid shear. Thus these long chain polymers lose their effectiveness when subjected to the increasing shear which is often present in the stock as it proceeds to the head box distribution header. Other additives such as fillers would ideally not be uniformly distributed through the thickness or the z-direction of the paper web but rather be concentrated at the surfaces. This is not possible with current methods employing a single headbox and single slice.
Multi-ply webs are known to be formed employing headboxes wherein the header is divided into sections allowing stocks of different types to be simultaneously injected through a single slice to form a multi-ply web. However, these systems are designed to give webs with distinct fiber contents rather than a uniform fiber content with varying amounts of chemical additives or fillers. Further, such devices may have difficulties employing the stock dilution method discussed above in two or more headers simultaneously.
What is needed is an apparatus for varying the chemical and filler additives concentrations in the z-direction of a paper web.